MSD 85703 Installation Instructions Manual

MSD Pro-Billet Small Diameter

Chevrolet V8 Distributor

PN 8570/85703

ONLINE PRODUCT REGISTRATION: Register your MSD product online. Registering your product
will help if there is ever a warranty issue with your product and helps the MSD R&D team create
new products that you ask for! Go to www.msdperformance.com/registration.

Important: Read these instructions before attempting the installation.

Parts Included:

1 - Pro-Billet Distributor
1 - Rotor, PN 8467
1 - Distributor Cap, Red PN 8431 or Black PN 84313
1 - Wire Retainer
2 - 1.5" Self Tapping Screws
2 - Panhead Phillips Screws

WARNING: Before installing the MSD Distributor, disconnect the battery cables. When discon-

necting the battery cables, always remove the Negative (-) cable rst and install it last.

1 - Advance Kit
1 - Gasket
1 - Tube of Gear Lubricant
2 - O-Rings

Note: The terminals of this Distributor require spark plug style terminals. You may need to change

the terminals and boots of your wires. MSD offers two kits, PN 8849 or PN 8848 that are supplied
with nine boots and terminals.

Note: If the gear is ever replaced, MSD Gear (PN 8531) is required for replacement due to the .500"

diameter shaft.

TIMING FUNCTIONS

Before continuing with the installation, here are a few denitions you should be aware of:

Initial Timing: This is the base timing (also referred to as idle timing) of the engine before the
centrifugal advance begins.

Centrifugal Advance: The centrifugal (or mechanical) advance mechanism is made up of weights,
springs, advance cams, and an advance stop bushing. The amount and rate of advance that your
distributor is capable of is determined by the centrifugal timing. If you ever wish to lock out the
centrifugal advance, refer to the centrifugal advance section.

Total Timing: This is the total of the initial timing plus the centrifugal advance added together.
Example: 10° Initial + 25° centrifugal = 35° Total Timing. (When checking Total timing, disconnect
the vacuum canister and plug the vacuum source.)

Note: MSD Distributors are supplied with the heavy (slow) advance springs installed. This is to prevent

detonation in certain applications. Review the information on pages 2-4 to determine the best
advance curve for your application.

MSD • WWW.MSDPERFORMANCE.COM • [phone] • FAX [phone]

2 INSTALLATION INSTRUCTIONS

CHOOSING AN ADVANCE CURVE

The function of the advance curve is to match the ignition timing to the burning rate of the fuel and
speed (rpm) of the engine. Any factor that changes the burning rate of the fuel or the engine speed
can cause a need for an ignition timing change. Figure 1 shows some of the factors that will affect
engine timing.

FACTOR Advance Timing Retard Timing
For For

Cylinder Pressure Low High
Vacuum High Low
Energy of Ignition Low High
Fuel Octane High Low
Mixture (Air/Fuel) Rich Lean
Temperature Cool Hot
Combustion Chamber Shape Open Compact
Spark Plug Location Offset Center
Combustion Turbulence Low High
Load Light Heavy

Figure 1 Ignition Timing Factors.

As you can see from the chart, most factors will change throughout the range of the engine operation.
The timing mechanism of the distributor must make timing changes based on these factors.

Example: An engine has 11:1 compression with a high energy ignition. With the specications given,
you will have to retard the timing for the high compression and high energy ignition. By comparing
the engine’s specications against the chart, a usable timing guideline can be found. Engines with a
combination of items from both columns will require a timing that is set in the mid range.

Obviously a full technical explanation of correct ignition timing would be very complicated. The best
way to arrive at a suitable ignition curve for your engine is to use the Ignition Timing Factors Chart
as a guide and compare it to the Advance Graphs in Figure 4 until a suitable curve is found. When
selecting your advance curve, use detonation (engine ping) as an indicator of too much advance, and
a decrease in power as an indicator of too little advance.

TIPS ON SELECTING AN ADVANCE CURVE

• Use as much initial advance as possible without encountering excessive starter load.

• Start the centrifugal advance just above the idle rpm.

• The starting point of the centrifugal advance curve is controlled by the installed length and
tension of the spring.

• How quickly the centrifugal advance (slope) comes in is controlled by the spring stiffness. The
stiffer the spring, the slower the advance curve.

• The amount of advance is controlled by the advance bushing. The bigger the bushing, the

smaller the amount of advance.

CENTRIFUGAL ADVANCE CURVE

SELECTING THE ADVANCE SPRINGS

The rate, or how quick the advance comes in is determined by the type of springs which are installed
on the distributor. The MSD distributors are equipped with two Heavy Silver springs installed. These
will give you the slowest advance curve possible (Figure 2). The parts kit contains two additional sets
of springs which can be used to match the advance curve to your particular application. Refer to the
Spring Combination Chart (Figure 3) for combinations that can be achieved.

MSD • WWW.MSDPERFORMANCE.COM • [phone] • FAX [phone]

INSTALLATION INSTRUCTIONS 3

To change the springs, remove the cap and rotor and use needlenose pliers to remove the springs.

To change the springs, remove the cap and rotor and use needlenose pliers to remove the springs.
Be sure the new springs seat in the groove on the pin.

Be sure the new springs seat in the groove on the pin.

Timing Curve From Factory

Figure 2 The Factory Equipped Curve.

SPRING COMBINATION RATE OF ADVANCE FIGURE 4

2- Heavy Silver SLOWEST A
1- Heavy Silver B
1- Light Blue
1-Heavy Silver C
1-Light Silver
2- Light Blue D
1- Light Silver E
1- Light Blue
2- Light Silver FASTEST F

Figure 3 Spring Combination Chart.

MSD • WWW.MSDPERFORMANCE.COM • [phone] • FAX [phone]

Figure 4 Advance Curves.